Distributed process control systems, like those used in chemical, petroleum or other processes, typically include one or more process controllers and input/output (I/O) devices communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital or combined analog/digital buses, or via a wireless communication link or network.
A process controller (sometimes referred to as a “controller”), which is typically located within the plant or other industrial environment, receives signals (sometimes referred to as “control inputs”) indicative of process measurements and uses the information carried by these signals to implement control routines that cause the controller to generate control signals (sometimes referred to as “control outputs”) based on the control inputs and the internal logic of the control routines. The controllers send the generated control signals over buses or other communication links to control operation of field devices. In some instances, the controllers may coordinate with control routines implemented by smart field devices, such as Highway Addressable Remote Transmitter (HART®), WirelessHART®, and FOUNDATION® Fieldbus (sometimes just called “Fieldbus”) field devices. Moreover, in many cases, there may be plant or other industrial equipment that operates in the plant or other industrial setting to perform some function that is not under direct control of the process controller, such as vibration detection equipment, rotating equipment, electrical power generating equipment, etc.
The field devices that are typically associated with controller, which may be, for example, valves, valve positioners, switches, and transmitters (e.g., temperature, pressure, level and flow rate sensors), are located within the process environment and generally perform physical or process control functions. For example, a valve may open or close in response to a control output received from a controller, or may transmit to a controller a measurement of a process parameter so that the controller can utilize the measurement as a control input. Smart field devices, such as field devices conforming to the well-known Fieldbus protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within a controller. Field devices may be configured to communicate with controllers and/or other field devices according to various communication protocols. For example, a plant may include traditional analog 4-20 mA field devices, HART® field devices, Fieldbus field devices, and/or other types of field devices.
The process controllers receive signals indicative of process measurements made by sensors or field devices and/or other information pertaining to the field devices and execute a controller application that runs, for example, different control modules that make process control decisions, generate control signals based on the received information, and coordinate with the control modules or blocks being performed in the field devices. The control modules in the controller send the control signals over the communication lines or links to the field devices to thereby control the operation of at least a portion of the process plant or system.
Information from the field devices and the controller is usually made available over a data highway to one or more other hardware devices, such as operator workstations, personal computers, or computing devices, data historians, report generators, centralized databases, or other centralized administrative computing devices that are typically, but not always, placed in control rooms or other locations away from the harsher plant environment. Each of these hardware devices typically, though not always, is centralized across the process plant or across a portion of the process plant. These hardware devices run applications that may, for example, enable an operator to perform functions with respect to controlling a process and/or operating the process plant, such as changing settings of the process control routine, modifying the operation of the control modules within the controllers or the field devices, viewing the current state of the process, viewing alarms generated by field devices and controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, keeping and updating a configuration database, etc. The data highway utilized by the hardware devices, controllers, and field devices may include a wired communication path, a wireless communication path, or a combination of wired and wireless communication paths.
As an example, the DeltaV™ control system, sold by Emerson Process Management, includes multiple applications stored within and executed by different devices located at diverse places within a process plant. A configuration application, which resides in one or more operator workstations or computing devices, enables users to create or change process control modules and download these process control modules via a data highway to dedicated distributed controllers. Typically, these control modules are made up of communicatively interconnected function blocks, which perform functions within the control scheme based on inputs thereto and which provide outputs to other function blocks within the control scheme. The configuration application may also allow a configuration designer to create or change operator interfaces which are used by a viewing application to display data to an operator and to enable the operator to change settings, such as set points, within the process control routines. Each dedicated controller and, in some cases, one or more field devices, stores and executes a respective controller application that runs the control modules assigned and downloaded thereto to implement actual process control functionality. The viewing applications, which may be executed on one or more operator workstations (or on one or more remote computing devices in communicative connection with the operator workstations and the data highway), receive data from the controller application via the data highway and display this data to process control system designers, operators, or users using the operator interfaces, and may provide any of a number of different views, such as an operator's view, an engineer's view, a technician's view, etc. A data historian application is typically stored in and executed by a data historian device that collects and stores some or all of the data provided across the data highway while a configuration database application may run in a still further computer attached to the data highway to store the current process control routine configuration and data associated therewith. Alternatively, the configuration database may be located in the same workstation as the configuration application.
As noted above, operator display applications are typically implemented on a system wide basis in one or more of the workstations and provide displays to the operator or maintenance persons regarding the operating state of the control system or the devices within the plant. Typically, these displays take the form of alarming displays that receive alarms generated by controllers or devices within the process plant, control displays indicating the operating state of the controllers and other devices within the process plant, maintenance displays indicating the operating state of the devices within the process plant, etc. These displays are generally configured to display, in known manners, information or data received from the process control modules or the devices within the process plant. In some known systems, displays have a graphic associated with a physical or logical element that is communicatively tied to the physical or logical element to receive data about the physical or logical element. The graphic may be changed on the display screen based on the received data to illustrate, for example, that a tank is half full, to illustrate the flow measured by a flow sensor, etc.
Traditional analog 4-20 mA field devices communicate with a controller via a two-wire communication link (sometimes called a “loop” or “current loop”) configured to carry a 4-20 mA DC signal indicative of a measurement or control command. For example, a level transmitter may sense a tank level and transmit via the loop a current signal corresponding to that measurement (e.g., a 4 mA signal for 0% full, a 12 mA signal for 50% full, and a 20 mA signal for 100% full). The controller receives the current signal, determines the tank level measurement based on the current signal, and takes some action based on the tank level measurement (e.g., opening or closing an inlet valve). Analog 4-20 mA field devices typically come in two varieties including four-wire field devices and two-wire field devices. A four-wire field device typically relies on a first set of wires (i.e., the loop) for communication, and a second set of wires for power. A two-wire field device relies on the loop for both communication and power. These two-wire field devices may be called “loop powered” field devices.
Process plants often implement traditional 4-20 mA systems due to the simplicity and effectiveness of the design. Unfortunately, traditional 4-20 mA current loops only transmit one process signal at a time. Thus, a set-up including a control valve and a flow transmitter on a pipe carrying material may require three separate current loops: one for carrying a 4-20 mA signal indicative of a control command for the valve (e.g., to move the valve to 60% open); a second for carrying a 4-20 mA signal indicative of the valve's actual position (e.g., so that the controller knows the degree to which the valve has responded to control commands); and a third for carrying a 4-20 mA signal indicative of a measured flow. As a result, a traditional 4-20 mA set-up in a plant having a large number of field devices may require extensive wiring, which can be costly and can lead to complexity when setting up and maintaining the communication system.
More recently, the process control industry has moved to implement digital communications within the process control environment. For example, the HART® protocol uses the loop DC magnitude to send and receive analog signals, but also superimposes an AC digital carrier signal on the DC signal to enable two-way field communications with smart field instruments. As another example, the Fieldbus protocol provides all-digital communications on a two-wire bus (sometimes called a “segment” or “Fieldbus segment”). This two-wire Fieldbus segment can be coupled to multiple field devices to provide power to the multiple field devices (via a DC voltage available on the segment) and to enable communication by the field devices (via an AC digital communication signal superimposed on the DC power supply voltage). Generally speaking, because the connected field devices use the same segment for communication and are connected in parallel, only one field device can transmit a message at any given time over the segment. Accordingly, communication on a segment is coordinated by a device designated as a link active scheduler (LAS). The LAS is responsible for passing a token between field devices connected to the segment. Only the device with the token may communicate over the segment at a particular time.
These digital communication protocols generally enable more field devices to be connected to a particular communication link, support more and faster communications between the field devices and the controller, and/or allow field devices to send more and different types of information (such as information pertaining to the status and configuration of the field device itself) to the process controller and other devices in or connected to the control network. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network.
Regardless of the communication protocol utilized, field devices may require on-site setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair.
Generally speaking, configuration and testing of field devices are performed on location using a handheld maintenance tool, such as a portable testing device (“PTD”). Because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a PTD rather than using a full configuration and testing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device.
Regardless of the communication protocol utilized, field devices may require setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair. Operators use industrial computing devices to configure, troubleshoot, calibrate, analyze and perform other operations on process control devices (e.g., controllers, field devices, etc.). In some known systems, an operator may conduct a number of analyses and/or management of a field device using a stationary industrial computing device (e.g., a personal computer, workstations, etc.) to perform asset management, vibration management, industrial computing device fleet management, etc.
On the other hand, configuration and testing of field devices may be performed on location using a portable industrial computing device, such as a handheld field communicator, calibrator, portable testing device (“PTD”), etc. An operator may physically attach the portable industrial computing device to a field device and may then communicate with the field device via the portable industrial computing device for diagnostics, to change the configuration, to calibrate the field device, etc. For example, because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a PTD rather than using a full configuration and testing device, such as a stationary industrial computing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device.
When a user, such as a service technician, performs maintenance testing and/or communications with a field device, the PTD is typically communicatively connected to a communication link (e.g., a current loop or Fieldbus segment) or directly to a field device (e.g., via communication terminals of the field device). The PTD initially attempts to communicate with the field device, such as by sending and/or receiving digital communication signals along the loop or segment using the communication protocol supported by the field device. If the current loop or segment is in proper operating condition, the communications signals may be sent and/or received without problem. However, if the loop, segment, or field device contains an electrical fault, such as a short or a break, communications may be impeded, and it may be necessary to diagnose the loop, segment, and/or field device to identify the fault.
When such a fault is identified, a technician might need to use a variety of other tools to test the field device and/or communication link. For example, the technician may need to use a portable power supply to power an isolated field device. The technician may need to power an isolated field device, for example, when the field device loses power due to a plant-wide power outage or due to an issue with a local power supply. As another example, the technician may simply need to take a field device offline for troubleshooting in order to avoid negatively effecting other field devices and the rest of the process control system. The technician may also need to carry a multi-meter to measure the current, voltage, resistance, impedance, etc. available on a segment or loop, etc. Each of these tools can take up a fair amount of space, and may be inconvenient for a technician to carry in the field. To address this problem with carrying multiple tools, manufacturers have developed PTDs that include a power supply for providing power to a HART loop. Unfortunately, these powered PTDs are typically incapable of providing power to Fieldbus field devices. Further, typical portable power supplies and powered PTDs often fail to comply with Intrinsic Safety (IS) standards, and thus cannot be safely used in hazardous areas (e.g., environments or atmospheres that are potentially explosive due to the presence of explosive gas or dust).
Still further, if a field device is located in a hazardous area, the technician may need to verify that each of his or her tools operates in an intrinsically safe manner. Thus, when in a hazardous area, a technician's tools may need to comply with IS standards to ensure safe operation. Generally speaking, IS standards impose restrictions on electrical equipment and wiring in hazardous environments to ensure that the electrical equipment and wiring does not ignite an explosion. To comply with IS standards, electrical equipment generally needs to be designed with two core concepts in mind: energy limitation and fault tolerance. In any event, the requirement for IS compliance in some uses has led to the development of another set of field maintenance tools that may work with one of the field device protocols mentioned above, or others, but that is also IS compliant.
Likewise, as mentioned above, most process plants and other industrial settings (such as oil well drilling platforms, pumping stations, etc.) include other industrial equipment, such as rotating equipment, power generating or conversion equipment, vibration analyzer equipment, etc. that needs to be set up, configured, and maintained within the plant or industrial setting. A still further set of field maintenance tools may be needed to support this equipment, including configuring the equipment, testing the equipment, etc.
These types of industrial computing devices often need to communicate and exchange data in a secure and reliable manner, with proper authorization, and this need is rapidly increasing to support ever increasing capabilities and applications, which highlights a number of specific needs related to communications and data exchange within the process control system (i.e., in-plant) separate from the data highway of communications between a control room and process control devices. Although information technology (IT) infrastructure, such as Transmission Control Protocol/Internet Protocol (TCP/IP), authentication, Active Directory, etc., exists in these industrial settings, additional mechanisms and services are needed to provide additional security and access control authorization. Also, addition mechanisms and services are needed to support the communications, messaging, data transfer, and other capabilities of additional process control system applications.